Um, wouldn't mixing magic and antimagic make giant magic explosions?
Actually, the interesting thing is, no.
Think of how matter and anti-matter interact. Yes, they make giant explosions; because they're matter, and all of the energy that makes them up is being turned into, well, energy, rather than the matter it was before. But what is magic? Magic is energy, right? So it can't annihilate itself by turning into more energy, now can it? Instead, it probably either turns into minute specks of matter, something we don't even have the ability to comprehend, or perhaps a different form of energy like neutrinos.
He sounds like a blank. Like a hole in magic. I can accept it as "He is extremely hard to directly attack with supernatural powers". Having the shared pool works - it would take serious effort to overcome the effect on himself. I'd suggest you specify just what sort of spells he can cast, though.
Play around with it a bit. It's almost there, but something doesn't quite work.
Aye, I still am not quite happy with it, either. As for the blank part; I was considering that, and I'm not sure if that's what I want to go with, but if I do/if something similar occurs, such as the antimagick(and because it probably infuses him and his equipment, would effectively negate most magic that came into contact with it, although if there was enough it would overwhelm it and so forth), and ANYWAY, if I did want something like that, it would probably be a matter of skill on behalf of the affecting party; he could never be attacked
directly, but if they were clever and figured it out or knew before hand, and had the skill to do so, they could probably make small adjustments so that the spell became simply a series of non-magical events before it hit him; instead of a fireball that explodes when it hits him, an ultra-dense packet of super-heated gas and particles that is held in place by magic, thus exploding when it hits him because of the release. Needless to say, things such as Wall of Force and Magic Missile would have no effect.
But I think I might be more likely for him simply to be really good at warding, counterspells, mana drain, silence, disrupting focus, all that good stuff. Possibly enchantments to some small degree, to perform upkeep on his equipment(his armor would probably be really heavily warded, of course).